Once you attend basic training you will find that your muscles will develop more (which you will not see until after), and this will of course be one aspect where the schools you attend will contribute to your physical makeup.

In the mean time, use a balanced physical training routine, and a proper diet - with protien supplements after a workout if you feel it works for you, and you should see a few pounds show after each 3 to 4 week cycle of your training plan. There are a lot of resources available to you either online, in this forum, or through other commercial outlets to help you find a plan that will help you focus not only on body strength, but endurance and stamina as well.